1. Field of the Invention
The present invention relates to a process for producing polyalkyl-substituted aromatic aldehydes having three or more C1-C3 alkyl groups which are useful as a raw material or an intermediate material for the production of medicines, agricultural chemicals, perfumes, etc.
2. Description of the Prior Art
Gattermann-Koch reaction has been well known in the art as a production method of an alkyl-substituted aromatic aldehyde by formylating an alkyl-substituted aromatic compound with carbon monoxide in the presence of a catalyst such as a hydrogen chloride-aluminum chloride catalyst. In this reaction, the reaction product mixture is generally treated with water to separate the reaction product and the catalyst, this making the regeneration of the catalyst extremely difficult. Further, a large amount of wastes are produced by hydrolysis to unfavorably increase disposal costs.
Modified Gattermann-Koch reactions using hydrogen fluoride and boron trifluoride as the catalyst are disclosed in U.S. Pat. No. 2,485,237, Japanese Patent Publication No. 39-29760, Japanese Patent Application Laid-Open No. 56-99433, etc. Since hydrogen fluoride and boron trifluoride used as the catalyst in the proposed modifications show a high vapor pressure, the hydrolysis is not needed to separate the reaction product and the catalyst, this allowing the catalyst to be recycled and reused. Therefore, the proposed modifications provide an industrially excellent process for the production of aromatic aldehydes.
In the formylation of the alkyl-substituted aromatic compounds in the presence of the hydrogen fluoride/boron trifluoride catalyst, the formylation mainly occurs at the para-position with respect to the alkyl substituent to give p-alkyl aromatic aldehydes. However, if the alkyl-substituted aromatic compound has, as in the case of mesitylene or isodurene, alkyl groups on both the carbon atoms adjacent to the site to be formylated, the formylation reaction thereof is extremely slow as compared with the formylation of other alkyl-substituted aromatic compounds. For example, “Journal of Japan Petroleum Society”, Vol. 20, pp. 655 to 661 (1977) teaches that toluene is formylated under mild conditions and in a short reaction time (under carbon monoxide pressure of 30 atm (2.9 MPa) for 20 min) with a yield as high as 94%, whereas the formylation of 1,3,5-trimethyl benzene (mesitylene) provides only 79% yield even under severe conditions of a carbon monoxide pressure of 200 atm (19.6 MPa) and a long reaction time of 1.5 h.
U.S. Pat. No. 2,485,237 discloses that the conversion of mesitylene is 82.3% in the formylation of 0.3 mol of mesitylene in the presence of 1.0 mol of hydrogen fluoride and 1.0 mol of boron trifluoride under a pressure of 100 to 730 atm (9.8 to 71.5 MPa) for 11 min. The pressure range proposed therein is too broad, but the formylation of mesitylene appears to require a considerably high pressure.
Thus, the formylation of the alkyl-substituted aromatic compound having alkyl groups on both the carbon atoms adjacent to the site to be formylated requires an extremely high carbon monoxide pressure of 10 MPa or higher and/or a very long reaction time to make the process industrially disadvantageous. In particular, the use of a pressure as high as 10 MPa or more in extremely strong corrosive conditions due to the hydrogen fluoride/boron trifluoride catalyst is industrially less practicable.